Conventionally, there has been techniques related to a callback system based VPN (Virtual Private Network) connection using an electronic mail delivery system from a terminal carried outside a company office.
For example, Japanese Patent Publication JP2004-280595A (Patent document 1), and Domestic re-publication of PCT international publication for WO2006/011464 (Patent document 2) disclose related techniques.
An authentication method according to Japanese Patent Publication JP2004-280595A (Patent document 1) is a method that makes a callback system based VPN connection. In order to realize the method in Japanese Patent Publication JP2004-280595A, each of a client device and a server device that make the VPN connection based on the callback authentication method requires an SMTP (Simple Mail Transfer Protocol) client function and a POP (Post Office Protocol) client function. On the basis of such an electronic mail delivery system, delivery/receipt of authentication information used for the VPN connection is achieved.
However, this technique cannot make the VPN connection in a situation or environment where the SMTP or POP cannot be used. That is, in a case where the electronic mail delivery system is not present, or cannot be used, such a connection method cannot be carried out.
Also, in an environment where IT infrastructure is not sufficiently constructed, the electronic mail delivery system serves as a bottleneck. In such a case, it is known that there is a case that the communication times out.
For this reason, there is required an access method without use of the electronic mail delivery system. For example, it is required that, without use of the electronic mail delivery system, authentication information on a client side terminal is delivered to a VPN server device present on a company infrastructure network.
However, such means that delivers the authentication information from the client device to the VPN server device through a direct communication connection cannot be used. The reason is because in order to make the direct communication, it is additionally necessary to place the VPN server device in a DMZ (DeMilitarized Zone), and make settings for communication to a firewall. This is contradictory to the advantage in Japanese Patent Publication JP2004-280595A (Patent document 1).